1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to optical systems for microscopic imaging, inspection and/or lithography applications.
2. Description of the Related Art
Many optical systems and electronic systems are available to inspect surface features of a specimen for defects, including specimens such as a semiconductor photomask or partially fabricated integrated circuit. Defects on such specimens may take the form of particles randomly localized on the specimen surface, scratches, process variations, repeating pattern defects, and so forth. Techniques and devices for inspecting specimens for these microscopic defects are generally available in the art and are embodied in various commercially available products, including those available from KLA-Tencor Corporation of San Jose, Calif., the assignee of the present application.
The aim of virtually any type of inspection system or technique is to rapidly and efficiently detect defects. With smaller and smaller features on specimen surfaces and the use of new materials and new manufacturing processes, detection of new and finer defects is required. It is also preferable to rapidly inspect a specimen surface in as short an amount of time as possible, from loading the specimen to removing it from the inspection position and characterizing the defects. Such speed requirements in the presence of smaller features mandates continuous improvements in the available techniques to accurately and adequately find specimen problems.
Inspection systems are available for wafer inspection, while still others target photomask inspection. The inspection systems currently available are highly complex, requiring a sophisticated combination of light source, illumination, imaging, positioning, automatic focusing, image sensor, data acquisition, and data analysis subsystems. A complete change in the inspection system may be required to allow the detection of new and smaller defects on a specimen.
Of the currently available systems, it should be noted that most use a non-pulsed light source from an arc lamp or a laser. A non-pulsed lamp or laser provides a relatively constant power to the specimen and is more easily implemented in a high speed inspection system. However, relatively constant and non-pulsed energy sources suffer from particular drawbacks. Short wavelengths have desirable characteristics for inspecting small defects. Few non-pulsed sources are available with the required power and brightness for high speed inspection at wavelengths below 300 nm. In addition, non-pulsed laser light sources do not produce light energy with relatively low spatial and temporal coherence, which can be a problem in certain circumstances. For this reason, these non-pulsed laser sources suffer from interference and speckle induced illumination discontinuities. Overcoming these problems requires time averaging of speckle patterns, such as by using a rotating ground glass plate.
It would therefore be desirable to have a system for inspecting a specimen that improves upon the systems previously available, and in particular for enabling inspection of specimens such as wafers and photomasks with short wavelength light that do not have the adverse effects associated with non-pulsed light sources.